- The Ultimate Desert Resource

Part I: Dwindling Southwest Water Resources
by Jay W. Sharp

 

This article marks the first of a three-part series on water issues in the Southwest. In this part, we will review the primary ground and surface resources available to our growing population. In the next two parts, we will examine 1) the problems emerging from a dwindling supply, growing demand and potential long-term drought and 2) the potential long-term solutions for looming water shortages.

Peak in the Colorado Rockies, the mountains that give rise to both the Colorado River and the Rio Grande.During the drought we have experienced over the past several years across much of the desert Southwest – including southern California, southern Nevada, southern Utah, southwestern Colorado, Arizona, New Mexico and western Texas – our water tables have fallen, wetlands have diminished, stream flows have dwindled and lakes have contracted.  

At the same time, our population has continued to grow rapidly, increasing the demand for water. Global warming has intensified, delaying autumn snowfalls and accelerating spring snow melts, raising autumn and spring temperatures, and drying out soils. Drought-stressed wood- and brushlands have been dying off, becoming fuel for massive wildfires. Forests have been retreating up mountain slopes, taking sanctuary in the wetter zones.  From the Pacific Coast eastward to the High Plains, Southwest communities – for instance, San Diego, Las Vegas, Tucson, Albuquerque and El Paso – confront potential shortages and must routinely impose water use restrictions. 

Some scientists have predicted that we can expect the drought to continue, bringing to the Southwest a dust bowl similar to the one the Southern Plains experienced during the Great Depression of the 1930s. Columbia University's Lamont-Doherty Earth Observatory scientists suspect that we may have already entered a long-term extreme drought. 

Intermittent mountain stream, issuing into desert basin after seasonal snows and rains in the higher elevations.Under the specter of a potential prolonged severe drought – perhaps much like those that have occurred in the Southwest more than a dozen times during the past eight centuries, according to the U.S. Geological Survey – federal, state and local government agencies have broadened efforts to reassess our region's water resources and consumption, identify and gauge potential problems, and develop innovative new management and technical solutions. It is the declining resources – both ground and surface waters – that signal change for our future.

Water Resources

Assessing our usable ground and surface water resources can be notoriously difficult. Aquifers, for instance, occur in widely scattered basin locations and at variable depths across the Southwest. Their waters vary in quality and accessibility. Their recharge processes and rates defy ready understanding. Stream flows and lake levels rise and fall largely in response to the amount and timing of snow and rain in the higher elevations, the onset of snow-melting temperatures in the spring, the locations and timing of the typically intense but scattered rainstorms of the Southwest, and the changeable needs for water drawn from ground or from surface resources. The resources, which largely define our lives in the Southwest, raise major challenges for water managers and technical experts. 

Ground Water

From the eastern escarpments of the Southern Plains westward across the Chihuahuan, Sonoran and Mojave Deserts, our ground water resources – or aquifers, or bolsons – reside within the sedimentary layers that fill the thinly vegetated basins between the mountain ranges. Originally, as the USGS said in its Internet site, Science for a Changing World, "Desert Basins of the Southwest," the ground water became accessible for human consumption when it issued from an aquifer as a spring or a seep, forming a stream or a wetland. Later, shallow ground water became available through hand-dug wells or through low-capacity drilled wells. More recently, the deeper ground water has been tapped by "high-capacity large-diameter drilled wells with turbine pumps…" 

End of a mountain stream, which is soaking into the gravels and sands of a desert basin and migrating toward an aquifer.

Most water flowing into a typical Southwest aquifer originates primarily as rainfall and snowmelt. Most of it flows from higher – and wetter – elevations through subsurface structural fractures or through surface streambeds into the margins of the basins. It seeps through the sedimentary layers – gravels, sands, silts and clays – to collect in the aquifers, sometimes moving at a rate of no more than several dozen feet in the course of an entire year. Gradually, it saturates the minute spaces, or "pores," between the grains of sedimentary layers below the water table.  While sedimentary layers may approach two miles in depth in some of our Southwest basins, the most accessible and highest quality water trapped within the layers typically lies within the upper few thousand feet. Eventually, sometimes after tens of thousands of years in an aquifer, water may reemerge as a spring, join with a neighboring aquifer, seep into wetlands or marshes, or bring nourishment to desert plants. 

Over time, even in our dry Southwest, large volumes of water may accumulate in a single aquifer. "For example," said the USGS, "in a basin in which the upper 1,000 feet of saturated sediments contain 15 percent removable water and having an average width and length of 15 miles and 25 miles respectively, the volume of water is 36 million acre-feet. [An acre-foot – a volume of water that would cover an acre of land to a foot in depth – equals about 326,000 gallons.] That volume of water [36 million acre-feet], if it could be practically pumped, would amount to a 2,000-year supply for a population of 100,000 having a per capita water use of 160 gallons per day." Collectively, prodigious volumes of water have collected in the Southwest's numerous aquifers. 

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